Laser shock peening or laser shock processing, as it also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece. Laser shock peening typically uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477, entitled "Laser Shock Processing"; and U.S. Pat. No. 5,131,957, entitled "Material Properties". Laser peening as understood in the art and as used herein utilizes a laser beam to produce a strong localized compressive force on a portion of a surface by producing an explosive force. The explosive force is produced by instantaneous ablation or vaporization of a painted or coated or uncoated surface beneath a force containing overlay which is typically a water curtain. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled "Laser Peening System and Method". Many methods and uses for using laser shock peening have been developed and could benefit from an accurate method of validation and non-destructive testing of effects of the process. Known methods include directing a beam of ultrasonic waves on to the surface of the material at a variable angle of incidence, detecting the energy of the corresponding reflected beam, and determining the critical angle of incidence at which the energy (or the amplitude of the reflected waves) appear to pass through a minimum. At the critical angle the acoustic energy within the material propagates along the surface of the material in what are defined as Rayleigh or Stonely waves which decrease logarithmically in amplitude with depth. Together with couplant velocities and Snell's Law, the critical angle can be used to calculate the propagation speed of the Rayleigh ultrasonic waves at the surface of the material. It is well known that these surface waves are useful in comparing material properties and changes in these properties.
An article in the ISA Transactions, Vol. 19, No. 2, published in 1980, entitled "Measurement of Applied and Residual Stresses" discloses an ultrasonic instrumentation system developed for nondestructive measurements of applied and residual stresses. The article discloses a system that measures time of flight of an ultrasonic wave through a material with a resolution of 0.1 nanoseconds and discusses how time of flight correlates directly with elastic stress levels. U.S. Pat. No. 4,210,028, entitled "Method And Apparatus For Ultrasonically Measuring Concentrations Of Stress", discloses an apparatus and method for ultrasonically measuring concentrations of stress in objects of interest. The apparatus includes an ultrasonic transducer array for propagating acoustic waves in the object along a plurality of determinable directions and from a plurality of determinable positions. Time of flight measurements are made and reconstructed into a map of the variations in acoustic velocity within the object. The changes in acoustic velocity are then mathematically converted into a map of stress concentration areas in the object of interest.
It is believed that laser shock peening imparts increased resistance to crack initiation and propagation resulting from cyclic fatigue by producing compressive residual stresses in the laser shock peened material. In addition to the compressive residual stress, the material may also be strengthened by the local work hardening due to plastic deformation at the site of the laser shock. Also, the orientation and degree of preferential crystallographic texture are expected to affect the fatigue damage tolerance of the part. All of these structural conditions affect the velocity of the different modes of acoustic wave propagation within a material. The wave propagation mode employed in this methodology is referred to as a Rayleigh wave, or Stonely wave. These are surface waves that decrease exponentially in amplitude with depth into the material. The velocity sensing methodology described in this disclosure employs Rayleigh waves generated by refraction, and senses their presence by a decrease, or "null" in the amplitude of the ultrasonic beam that is both reflected and re-radiated from the part's surface. It is well recognized that the Rayleigh or critical angle at the null may be used to determine and analyze absolute and relative material properties for evaluation and comparative purposes.
The Rayleigh wave may be generated by refraction of the incident acoustic beam at a liquid solid interface where the liquid, often water, is referred to as a couplant. The angle at which an incident longitudinal or compressional wave is refracted and converted to a Rayleigh wave propagating parallel with the surface of the material is referred to as the Rayleigh Critical Angle. Changes in the Rayleigh Critical Angle at which these surface waves are generated are an indicator of changes in the Rayleigh wave velocity and, therefore, an indicator of changes in material properties.
It has been found that the angle at which the Rayleigh Wave is generated is evidenced by a "sharp" decrease, or "null" in the intensity profile of the superimposed reflected and re-radiated acoustic beam. The depth of the null has been found to be a function of the attenuation within the material and of the wave length or frequency of the acoustic beam. There is a frequency related maximum null condition, and there is also a phase change associated with the received signal that is dependent on whether the frequency of the incident beam is above or below that associated with the maximum null. Because of these effects, simply measuring the changes in the amplitude of the received signal at some single fixed angle slightly smaller or larger than the Rayleigh Critical Angle (on the slope of the null) will result in indications colored by the effects of many material properties such as grain size, preferential crystallographic orientation or texture, work hardening, etc. and measurement related variables such as couplant temperature. To separate the effects of attenuation from properties related to residual stress and texture some means of directly determining the angular position of the null must be employed in a quick and effective manner suitable for process validation in a production environment. The prior art methods for determining critical angles are very time consuming and a more practical method for process evaluation and product validation is highly desirable. The present invention is directed towards this purpose. The present method provides a non-destructive material evaluation technique using ultrasonic waves and the determination of effective Rayleigh wave critical angles for use in quality control to evaluate the degree of change in properties due to materials processing operations such as laser shock peening. The effective Rayleigh wave critical angles of the present invention may not be exact but are sufficient for use in quality control to evaluate the degree of change in properties due to materials processing operations.